Zeta diversity of native (a) and alien (b) plant species. Zeta decline (rescaled between 0 and 1 for comparison; the x-axis is on a log-scale and stops at order 15 to highlight differences between assemblages of the four regions) (left) and the corresponding zeta ratio computing the retention rate (right). The x-axis of the zeta decline is on a log-scale for clarity. PolaPlai – Polish Plain, HighWeCa – High Western Carpathians, MatrWeCa – Matricum of Western Carpathians, Pannonian – Pannonian lowland.